1. Field of the Invention
The present invention relates to an aromatic liquid crystalline polyester resin which is excellent in film molding property (film forming property) and a resin composition thereof.
2. Description of the Related Art
Recently, films produced from various polymers are essential in daily life. For example, there have been developed a variety of films such as high strength films, high elasticity films, oxygen barrier films, conductive films, heat-resistant films, light shielding films and the like. Among other, oxygen barrier films or water vapor barrier films are widely used in our life as wrapping materials irrespective of industrial use or private use.
As the material having oxygen barrier property and water vapor barrier property, liquid crystalline polymers, particularly, a thermotropic aromatic liquid crystalline polyester (hereinafter, sometimes abbreviated as LCP) is noticed, and film forming from this polyester is in investigation. For example, there are reported films made of a liquid crystalline copolyester composed of PET/p-hydroxybenzoic acid (hereinafter, sometimes abbreviated as POB) having an aliphatic chain in the main chain (Japanese Patent Application Publication (JP-B) No. 8-2974) or made of a semi-aromatic liquid crystalline polyester (JP-B No. 6-53383), and films made of a wholly aromatic liquid crystalline polyester (JP-A Nos. 7-323506 and 7-251438), and the like. However, in these investigations, it is difficult to form thin films since molecular orientation occurs in flow direction (MD) which is characteristic of LCP and anisotropy in mechanical strength occurs against the transverse direction (TD), therefore, it is difficult to obtain a practical LCP-based thin film having a thickness of 50 .mu.m or less.
Wholly aromatic LCPs often require a processing temperature of over 300.degree. C. excepting PET/POB copolyesters and polyesters obtained by copolymerization of different aromatic hydroxycarboxylic acids (for example, Vectra series), and heat-resistant wholly aromatic liquid crystalline polyesters having a temperature of deflection under load (TDUL) of over 250.degree. C. sometimes require a further higher processing temperature of 350.degree. C. or more, therefore, an expensive and specific molding machine is necessary.
As film forming methods from a wholly aromatic polyester which forms an optically anisotropic melt, an blown film forming method and a flat film forming method using a T die method are disclosed, for example, in JP-B Nos. 62-58378 and 63-33450 and the like. However, any of these methods needs a processing temperature of 340.degree. C. or more, and lowering of the processing temperature is still required.
A wholly aromatic liquid crystalline polyester which has a molecular weight for sufficient mechanical strength and can be molded at a low temperature (320.degree. C. or lower) is disclosed in JP-B No. 63-3888 and a semi-aromatic liquid crystalline polyester is disclosed in JP-A No. 61-102234 and the like. As film forming methods of the polyesters, a method for film forming at a high shear rate is described in JP-A No. 2-3430, a, film forming method using a ring die and the like is described in U.S. Pat. No. 4,975,312 and WO9015706. Any of them disclose a method for relaxing anisotropy of a liquid polyester by a special molding method, which has a limitation in film-thinning, and practical problems still remain. And also, cost performance should be further improved since a monomer having special chemical structure is used. JP-A No. 2-3430 describes melt strength as one of technical factors in film formation. This reference describes about an aromatic liquid crystalline polyester resin derived from only an aromatic hydroxycarboxylic acid unit, and the melt strength is described as a factor for producing a film at a high shear rate, and there is no suggestion regarding other aromatic liquid crystalline polyester resins.
Further, for lowering the melting point of an aromatic liquid crystalline polyester resin, for example, Brit. Polym. J. 154-162 (1980) describes a method for introducing a bent chain structure such as an aliphatic chain into the main chain, a method for introducing a substituent into an aromatic ring, a method for introducing m-orienting bent structure (isophthalic acid, resorcinol and the like), and a method for introducing crankshaft structure (e.g., 6-hydroxy-2-naphthoic acid, 2,6-dihydroxynaphthalene and the like). A wholly aromatic liquid crystalline polyester synthesized from an aromatic hydroxycarboxylic acid derivative introduced crankshaft structure is described in JP-B No. 63-3888, and put into practical use as a resin which can be molded at a processing temperature of 320.degree. C. or lower. However, a monomer having crankshaft structure is expensive, therefore there have been developed aromatic polyesters obtained from an aromatic hydroxycarboxylic acid, an aromatic dicarboxylic acid and an aromatic diol utilizing m-orienting bent structure (isophthalic acid, resorcinol and the like) which is advantageous in raw material cost. For example, such aromatic polyesters are described in JP-B Nos. 47-47870 and 52-24467, JP-A No. 1-152123 and the like, however, processing temperatures thereof are 320.degree. C. or more. JP-B No. 62-58378 describes that even if aromatic dicarboxylic acid components are all composed of isophthalic acid, the film forming temperature of T die is 360.degree. C.
LCP resin-based film materials having high gas barrier properties and processability at a low temperature of 320.degree. C. or lower still have a lot of problems for practical use.
Under such conditions, problems to be solved by the present invention, that is an object of the present invention is to provide a aromatic liquid crystalline polyester resin which has sufficient heat-resistance as a wrapping material and has improved low temperature processability, and can produce a film having excellent gas barrier property, and a resin composition thereof.